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Influence of Air Annealing on High Efficiency Planar Structure Perovskite Solar Cells Sonia R. Raga, Min-Cherl Jung, Michael V. Lee, Matthew R. Leyden, Yuichi Kato, and Yabing Qi* Energy Materials and Surface Sciences Unit (EMSS), Okinawa Institute of Science and Technology Graduate University (OIST), 1919-1 Tancha, Onna-son, Okinawa 904-0495, Japan S Supporting Information *
ABSTRACT: In the past few years, lead halide perovskite solar cell power conversion efficiencies have risen by using a wide variety of fabrication methods and just passed 20%. Perovskite solar cells are typically fabricated in a glovebox to strictly avoid any water exposure. A dry atmosphere significantly increases equipment and operational costs for industrial processes, so ambient perovskite fabrication will be less expensive and more attractive. In this work it is demonstrated that ambient annealing is comparable to annealing in dry N2. Perovskite films annealed in a standard dry N2 environment are compared with those annealed in ambient environment with 50% relative humidity. Solar cell devices were prepared with a planar structure configuration and annealed at one of three different temperatures (105, 115, or 125 °C) in either N2 or ambient air. For all temperatures, the average efficiencies for the devices annealed in air are higher than those annealed in dry N2. The highest efficiency achieved for air-annealed devices is 12.7%. Thus, good efficiency cells can be fabricated in an ambient environment, which facilitates mass production.
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INTRODUCTION Clean and renewable sources need to be developed to meet the increasing energy demand.1 Solar cell technology based on silicon has advanced considerably, but still suffers from high fabrication costs. In the last two decades, research on cheap and easily processed solar cells has flourished. Recently discovered perovskite-based solar cells are the most promising. Since the first reported perovskite device,2 the number of publications have exponentially increased, with numerous different ways to fabricate the devices.3,4 The preparation of nanostructured transparent conducting oxide, perovskite, and hole-transporting layers has been finely tuned to optimize charge separation and energy alignment, and also to increase coverage and crystallinity.5−10 Dozens of slight variations have been reported, but most of them have one common detail, i.e., to avoid any ambient exposure before completing the device because humidity is one of the possible causes for degradation of perovskite. Only a few authors have even reported annealing their perovskite films in air.11,12 One recent work from Zhou et al. suggested a mechanism that could enhance reconstruction of the perovskite films when annealed in air.13 The underlying mechanism here is analogous to the solvent-annealing process reported by Xiao et al.14 Highly hygroscopic methylammonium (MA) cation pulls moisture from the environment, which then partially dissolves the perovskite material and enlarges CH3NH3PbI3 (MAPbI3) crystals. In this work we compare the effects of perovskite annealing in a dry N2 glovebox versus an environment representative of © 2015 American Chemical Society
industrial conditions, i.e., open air with 50% humidity. Each sample was annealed at 105, 115, or 125 °C. We focused on planar configuration solar cells based on one-step solutionprocessed CH3NH3PbI3‑xClx perovskite films. In the planar architecture the perovskite coverage is more difficult to control than the solar cells with a mesoporous oxide layer. Low coverage may create shunt paths between the selective contact layers. However, by using the planar structure, we simplify the devices decreasing the overall fabrication cost and will also accentuate any degradation of samples in air. We followed a slightly modified recipe from Lee et al. for perovskite preparation, spin coating a methylammonium iodide (MAI) and PbCl2 solution 2.5:1 ratio on the TiO2 compact layer.5 With this fabrication method we obtained high perovskite coverage on TiO2 compact layer. We observed higher efficiency for the air annealed perovskite, which we attribute to growth of larger perovskite crystal grains and to complete evaporation of excess methylammonium lead chloride (MAPbCl3). The best performing solar cell achieved an efficiency of 12.7%, with a planar structure configuration, which is remarkably high for a nonoptimized device.15 We expect higher efficiencies after optimizing film thicknesses. Air annealing combined with a simple planar structure will promote the mass production of perovskite solar cells. Received: November 14, 2014 Revised: February 3, 2015 Published: February 3, 2015 1597
DOI: 10.1021/cm5041997 Chem. Mater. 2015, 27, 1597−1603
Article
Chemistry of Materials
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RESULTS AND DISCUSSION Perovskite Film Morphology. Higher annealing temperatures and air-annealing increase perovskite crystal grain size and film uniformity based on atomic force microscopy (AFM) analysis (Figure 1). Planar polycrystalline domains are 1−3 μm
(110), (220), (330), and (440) orthorhombic crystal structure peaks for CH3NH3PbI3.5 Higher annealing temperatures strengthen these peaks. The expanded scale shown in Figure 2b reveals additional XRD peaks that depend on the environment during annealing. Annealing in air produces XRD peaks indicative of PbI2, while annealing in nitrogen produces trichloride perovskite peaks. A minor XRD peak appear at 12.6° in spectra from airannealed samples. This is assigned to the (001) peak of PbI2 hexagonal crystals.16,17 The intensity of the PbI2 peaks progressively increases at higher annealing temperature. In XRD spectra from N2-annealed samples, minor peaks can be attributed to two different origins. The peaks at 15.6° and 31.5° can be assigned to cubic phase trichloride perovskite, CH3NH3PbCl3.18 The peaks at 14.0°, 28.3°, 42.9°, and 58.4° correspond to the (110), (220), (330), and (440) peaks for iodine-based perovskite but with a slight shift to lower diffraction angles. The gentle shift may result from a lattice distortion or strain in the MAPbI3 perovskite. Either two distinct MAPbI3 crystal structures, or, alternatively, strain from trace Cl incorporated in the MAPbI3 lattice could cause these additional peaks. Methylammonium Chloride Removal. We have often observed that white material deposits on the Al foil that covers the samples during perovskite annealing. More white material is found on the Al foil after annealing in air than annealing in N2. The white material was analyzed by XRF (Figure 3d) confirming the presence of chlorine but not iodine. It has been recently reported that methylammonium chloride (MACl) has a lower sublimation temperature than MAI, PbI2, or PbCl2.19 Although the temperatures employed in our
Figure 1. AFM images of the perovskite films on TiO2 compact layer prepared with different annealing conditions: (a) 105 °C in air, 92 nm roughness; (b) 115 °C in air, 99 nm roughness; (c) 125 °C in air 143 nm roughness; (d) 105 °C in N2, 59 nm roughness; (e) 115 °C in N2, 64 nm roughness; and (f) 125 °C in N2, 101 nm roughness.
larger in the air-annealed films than in the N2-annealed films. Interstitial grains 0.3 ± 0.1 μm fill the space between the large domains. Larger grains correlate directly to higher film roughness; root-mean-square (RMS) roughness provides a number for quantitative comparison of different films scanned by AFM in the same conditions. The measured RMS roughness was 90, 92, and 141 nm for samples annealed in air at 105, 115, and 125 °C, respectively; the corresponding values for N2annealed samples are 50, 57, and 80 nm, respectively. X-ray diffraction patterns for all the samples are shown in Figure 2. The main diffraction peaks of perovskite are found at 14.1°, 28.4°, 43.2°, and 58.9°, which can be assigned to the
Figure 3. Analysis of material released during perovskite annealing. (a) Position of glass substrates with top surface facing down toward the perovskite samples on the hot plate during annealing. PbI2 was spincoated on one substrate, but not on the other. (b) Picture showing the final color of the glass substrates. The substrate A was uncoated and not exposed to vapor from the perovskite samples. Substrate B was uncoated and exposed to vapor from the perovskites samples. White material deposited on substrate B. Substrate C was coated with PbI2 and turned red after 20 min exposure to vapor from perovskite samples that were being annealed. (c) XRD spectra taken on the B glass substrate and C substrate. The PbI2 peak is indicated in the plot: asterisks indicate the MAPbI3 perovskite peaks and green circles indicate the MAPbCl3 peaks. (d) XRF spectra of the above-mentioned substrates. Chlorine is confirmed for all the substrates except the blank (A). Iodine in the substrate B is below the detection limit of the instrument.
Figure 2. XRD spectra of the perovskite films prepared at different annealing conditions: (a) full-scale, (b) expanded view to show the low intensity peaks. Asterisks indicate the peaks corresponding to FTO and TiO2 substrate. 1598
DOI: 10.1021/cm5041997 Chem. Mater. 2015, 27, 1597−1603
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Chemistry of Materials
Figure 4. XPS data for (a) O 1s, (b) Cl 2p, (c) C 1s, (d) N 1s, (e) Pb 4f, and (f) I 3d core-level spectra. Significantly, we observed Cl in the N2 annealed sample, but no Cl element in the air annealed sample. C 1s and N 1s core-level spectra of both samples showed differences in chemical states or peak intensity.
annealed sample. Figure 4f shows the I 3d core-level spectra of both samples with only a single chemical state at 619.4 eV. Both peak intensity and fwhm of I 3d for the air-annealed sample are larger than those for the N2-annealed sample. The carbon and nitrogen betray a chemical change in the methylammonium ion of the perovskites. In the case of C 1s core-level spectra (Figure 4c), two different chemical states of 286.3 and 284.8 eV were found for both samples. The higher binding energy peak (higher oxidation state) is stronger in N2annealed samples, while the lower binding energy peak is stronger in the air-annealed samples. The N 1s core-level peak at 402.5 eV of the air-annealed sample is less intense than that of the N2-annealed sample (Figure 4d). The C 1s (286.3 eV) and N 1s (402.5 eV) core-level intensities of the air-annealed sample have a respective decrease of 49.9% and 52.2% relative to the N2-annealed film. Loss of nitrogen, carbon, and chlorine are consistent with losing MACl during annealing in air. In the case of Pb 4f core-levels of both samples (Figure 4e), we observed two chemical states at the binding energies of 138.7 and 136.8 eV. The binding energy of 138.7 eV is attributed to the Pb−I or Pb−Cl species.24−26 The second peak shifted 1.9 eV to the lower binding energies at 136.8 eV revealing the presence of metallic lead (Pb0).27 Previous works on MAPbI3 or MAPbIxCl3‑x also detected metallic Pb, but the origin was not conclusive.28,29 One possibility is beam damage to the samples during the measurements. Figure 4f shows the I 3d core-level spectra of both samples with only a single chemical state at 619.4 eV. The peak intensities and fwhm of Pb 4f and I 3d are larger in the airannealed sample. The optical band gap was determined to be 1.6 eV for both samples based on UV−vis measurements (Supporting Information Figure S1). Solar Cell Efficiency. We prepared solar cells from perovskite films that were annealed the same as samples for characterization above to relate the annealing environment to device performances. Table 1 summarizes the average of the photovoltaic parameters obtained from all the solar cells in each batch. One representative j−V curve for each batch is plotted in
work are much lower, MACl appears to sublime from the samples. We checked whether the white material is really MACl by attaching substrates of microscope glass to the Al foil facing down during annealing. One substrate was spin-coated with a layer of PbI2, while the other was uncoated glass. After exposure to the MACl from annealing samples, both substrates are compared to a blank, unexposed glass substrate in Figure 3b. After 20 min of exposure to the vapor from annealing samples, the PbI2 film on a hanging substrate turned red, while the uncoated substrate turned cloudy. XRD spectra (Figure 3c) confirmed the presence of both MAPbCl 3 andMAPbI3perovskite in the red film. XRF spectra confirmed Cl on the samples with (labeled C in Figure 3b) and without (B) the PbI2 layer, but not on an unexposed glass piece (A). Also, iodine in the sample B is below the detection limit of the instrument. This experiment directly confirms CH3NH3Cl depletion during the annealing of solution-prepared perovskite samples and demonstrates the formation of new perovskite from it. Recently, other groups published analysis of a film like our sample B.20,21 Spectroscopic Characterization. HRXPS analysis divulges the chemical states and relative composition of the perovskite films after annealing in different environments. Figure 4 shows the core-level spectra for the perovskite characteristic elements after compensating for the shift in the valence band maximum. The O 1s core-level peak at 533.1 eV is very weak for both samples (Figure 4a).22 The weak peak indicates minimal contamination from water or oxygen during the transfer of the samples from the glovebox to the XPS system and demonstrates that there is no water contamination after annealing in air. In addition, the negligible oxygen signal indicates that the perovskite film covers the underlying compact TiO2 layer without pinholes. The halide levels agree with preferential evolution of MACl that is enhanced when annealing in air. In Figure 4b, the Cl 2p3/2 core-level peak of the N2-annealed sample is located at 198.6 eV, corresponding to the Cl− anion.22,23 On the other hand, there is only a trace amount of Cl observed for the air1599
DOI: 10.1021/cm5041997 Chem. Mater. 2015, 27, 1597−1603
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The j−V curves of all solar cells have an “s-shape” near the open circuit voltage (Voc), which reduces the Voc and FF. The “s-shape” solar cell response decreases after preillumination with an open circuit (Supporting Information Figure S2). A 40 s time period maximized the Voc and FF. The “s-shape” remained and reduced the Voc for most of the N2-annealed samples after light-soaking but was nearly eliminated for airannealed samples. Previous works observed similar transient behavior and attributed it to light-induced halide diffusion in perovskite, or to dipole reorientation of MA+ in the perovskite lattice.33 The increased number of grain boundaries in the N2 annealed film is proposed to impede ion diffusion and produce the constant s-shape.34 We observed this behavior in perovskite solar cells prepared in our lab from MAI and PbI2 precursors, so the presence of MAPbCl3 is likely not the major cause of the s-shape. The hysteresis observed between forward and reverse scans is similar for both air and N2-annealed films regardless of the presence or absence of MAPbCl3 (Supporting Information Figure S2). This suggests that hysteresis depends primarily on bulk MAPbI3 perovskite. Proposed Interpretation. We have identified MAPbCl3 structurally and chemically in the N2 annealed samples but not in air annealed samples. Crystal growth in perovskites is reported to be driven by interfacial energy and supersaturation of the components.35 Low chloride solubility in CH3NH3PbIxCl3‑x has been suggested to induce segregation of the pure chloride-containing perovskite phase.32 Previous reports showed that MAPbCl3 formation depends on the MAI:PbCl2 ratio in the precursor solution.31 We propose that Cl− and I− perovskites have formed independently during solvent drying. MAPbCl3 supersaturation is induced by the almost negligible solubility of PbCl2 (320 g L−1) in the same solvent at room temperature. The equilibrium between the ions expressed by eq 1 allows excess PbCl2 to convert into MACl or vice versa, so that stoichiometric MAPbCl3 can precipitate.
Table 1. Photovoltaic Performance Parameters Extracted from j−V Measurements under Standard AM1.5 Illumination (1000 W m−2)a sample N2-105 °C air-105 °C N2-115 °C air-115 °C N2-125 °C air-125 °C
Voc [mV] 935 970 951 972 953 965
± ± ± ± ± ±
43 16 26 36 8 9
Jsc [mA cm−2] 13.1 17.2 19.8 18.9 16.6 18.7
± ± ± ± ± ±
3.0 1.3 0.7 0.7 1.0 1.3
FF [%] 50.6 48.1 58.8 61.6 53.0 61.2
± ± ± ± ± ±
7.8 5.0 6.5 2.9 3.5 1.4
η [%] 6.7 7.7 10.7 11.2 8.5
± 1.6 ± 1.3 ± 1.4 ± 0.8b ± 0.2 ±0.8c
a
The statistics correspond to 12 devices for the samples annealed at 105 and 115 °C, and 6 devices for the 125 °C. bMost efficient cell was 12.5%. cMost efficient cell was 12.7%.
Figure 5. The average efficiencies obtained for the air-annealed samples are slightly higher than the N2 annealed ones, regardless of the annealing temperature employed, suggesting that the 50% relative humidity does not degrade perovskite films during the annealing. We attribute the slightly higher efficiency to either, or possibly both, the larger grain size and the absence of MAPbCl3 perovskite in films annealed in air. Larger crystal sizes have been reported to enhance perovskite-based solar cell performance by reducing the number of grain boundaries and improving the charge extraction process.14 On the other hand, we expect that the larger band gap of MAPbCl3 perovskite could be detrimental for the solar cell efficiency hindering charge transport through the film. The reported values of valence and conduction band energy level for MAPbCl3 perovskite suggest that this material may block both electron and hole extraction depending on the position in the film.30 However, a wide range of results for photovoltaic performance are found in the literature for devices containing MAPbCl3 phase; in some works the chlorine phase is detrimental and in others no obvious effect was observed.13,31,32 Due to the differences in the perovskite films, a detailed study needs to be done in order to differentiate the effect of MAPbCl3 and crystal size in the final solar cell performance. Photogenerated current is slightly greater for the air-annealed samples and correlates to higher absorbance measured by UV− vis (Supporting Information Figure S1b). However, the direct relationship of jsc to the film composition is not straightforward, as other factors such as film thickness or crystallinity can also influence the photocurrent.
PbCl 2 + 2MAI ⇋ PbI 2 + 2MACl
(1)
Thus, higher ratio of PbCl2 in the precursor solution may lead to faster precipitation of chlorine species. MAPbI3 reaches supersaturation at much higher concentrations after most of the DMF has evaporated. Hence, for the N2 annealed samples, crystal growth will be driven by the rate of DMF evaporation, and MAPbCl3 and MAPbI3 will form in separate stages.
Figure 5. j−V curves for a representative solar cells; the scan was performed from 1.1 to 0 V at a scan rate of 0.2 V s−1. The solar cells were preexposed to illumination at open circuit conditions for 40 s. Hysteresis curves are found in the Supporting Information. 1600
DOI: 10.1021/cm5041997 Chem. Mater. 2015, 27, 1597−1603
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Highly crystalline MAPbI3 enhances the efficiency of the solar cells.
The process may be different in humid air. PbI2 is almost insoluble in water (0.76 g L−1) while PbCl2 solubility (10.8 g L−1) in water is similar to its solubility in DMF; thus, the precipitation rate of PbI2 will be increased by water from the air. We noticed that films change from transparent to black, immediately after being taken out of the glovebox and exposed to the air. Although this color change is not clearly understood, we expect hygroscopic DMF and methylammonium halide will absorb humidity from the air. We suggest that some PbI2 will precipitate immediately in the presence of water, and will further react with surrounding MAI to form MAPbI3, according to eq 2. The reaction of solid PbI2 with dissolved MAI has been observed in previous works such as the sequential deposition of perovskite.16 H 2O
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CONCLUSIONS We have demonstrated that the perovskite annealing in ambient air increases MAPbI3 perovskite crystal grain size and changes the composition of the film. MAPbCl3 is found to be formed spontaneously during DMF evaporation, but its formation is prevented in the presence of air during annealing, instead favoring MAPbI3 crystallization. Any water that might be adsorbed before or during the annealing process is completely removed after the annealing, as confirmed by XPS, so it will not affect the solar cell degradation once the device is assembled. Larger crystal size and the absence of MAPbCl3 are associated with an overall higher solar cell performance for the devices annealed in air. High efficiencies up to 12.7%, as well as higher Voc and FF, are obtained for samples annealed in 50% relative humidity. These results demonstrate that completely dry atmosphere is unnecessary for achieving proper perovskite formation. Simple and low-cost air annealing may help promote mass-production of perovskite solar cells.
MAI
PbI 2(DMF) ⎯⎯⎯→ PbI 2(s) ⎯⎯⎯→ MAPbI3(s)
(2)
The precipitation of PbI2 will reduce the available Pb in solution to form MAPbCl3 leaving an excess of unreacted MACl in the film. Yu et al. reported the disappearance of the MAPbCl3 XRD peak at the same time that the MAPbI3 peak was increased, after exposing a 15 min-annealed film to air.36 This effect on a DMF-free film suggests that water dissociates MAPbCl3 and promotes MAPbI3 formation in a “wet” film. Briefly, MAPbCl3 is spontaneously formed in the N2annealed films, but its formation is suppressed in humid conditions. The higher thermal stability of MA+ and Cl− in the form of MAPbCl3 compared to MACl can explain why Cl is observed in the films annealed in N2 and not in the films that are air-annealed at the same temperature.19 In addition, the high hygroscopicity of MA+ species will slow down the perovskite formation in air, because evaporation needs to take place before MAPbI3can form. This effect is analogous to the previously reported “solvent annealing” which increases the perovskite crystal size.13,14 Such an effect will be also valid for a system containing only MAI and PbI2. The system with only MAI and PbI2 is independent of the precipitation equilibrium explained above. Because of this difference, comparison of MAI + PbI2 films annealed in different environments could allow the effect of crystal grain size and the effect of MAPbCl3 to be considered separately. Several previous works reported an enhancement of photovoltaic performance in perovskite formed by mixedhalide precursors, relative to pure iodide precursors. One hypothesis is the formation of a mixed-halide perovskite with enhanced electrical properties. However, there is a lack of data corroborating the actual presence of chlorine in the perovskite.37 Another hypothesis is that the function of Cl− is to facilitate the sublimation of excess MA+ because MACl sublimates approximately 70 °C lower temperature than MAI.36 On the basis of our proposed process, Cl− from PbCl2 reacts with excess of MAI according to the solubility equilibrium from eq 1, to promote crystallization of MAPbI3. Different solvents or even the water adsorption in different concentrations will modify the solubility equilibrium. In addition, in air-annealed samples, PbI2 crystal peaks are found in the XRD spectra. Previous works assigned the formation of PbI2 phase to MAPbI3 degradation by thermal decomposition.38 However, depletion of iodine was not observed (Figure 3). Furthermore, MAPbI3 and PbI2 XRD peaks in Figure 2 are both enhanced at higher annealing temperatures. We suggest that PbI2 forms parallel to enhanced crystallization of MAPbI3.
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EXPERIMENTAL SECTION
Perovskite Precursor Preparation. Methylammonium iodide material was synthesized according to a slightly modified literature procedure.5 Hydroiodic acid was gradually added to methyl amine ethanol solution kept stirring in an ice-bath. Ethanol and water from the mixed solution were evaporated using a rotary evaporator (BUCHI, Rotavapor R3̅); crystals were formed by evaporation. The yellow resulting crystals were redissolved in hot ethanol, cooled at 5 °C in a refrigerator for recrystallization, and subsequently filtered and washed with tetrahydrofuran and diethyl ether resulting in white crystal powder. MAI crystals were dried and kept in a N2 glovebox (
